Detector for locating underground cables and faults therein using high-powered electromagnet

ABSTRACT

A method and an apparatus for locating from above the ground or water the position of or cable fault in a cable buried underground or underwater. An alternating magnetic field generated by high-powered electromagnet or a rotating magnet induces an indicator signal current in the wires of a cable to be examined. At one or both ends of the cable, the indicator signal is monitored by means of a receiver connected to the cable wires. The received and amplified indicator signal is transmitted by a radio phone back to the point of transmission, whereat the indicator signal is listened to for detecting the location of a cable and its possibly faulty section. According to the invention, a sufficient direct voltage is supplied between the wires of the cable. This creates a current whose loop closes through a faulty location and the indicator signal is separated from the current produced by the direct voltage.

The present invention relates to a method and an apparatus forlocalizing the short circuit of a pair of wires in an underground cabledirectly from ground level without having to dig "listening holes" alongthe route of a cable. The same way, a method and an apparatus of theinvention can be used for localizing the route or trail of a cabledirectly from ground level.

In the prior art cable-fault localizing equipment, an indicator signalis transmitted along a faulty twin cable and the indicator signal ismonitored on the ground. One of the drawbacks of this prior art methodis the ohmic resistance as well as overheating of a telephone cablerelative to other intact twin cables and also the limited maximumcurrent resisting capacity of a twin cable. The normal telephone currentin a standard cable is 20 mA. If this current is exceeded, the resultingdrawback is that the present indicator signal is induced in adjacenttwin wires and by-pass the faulty location.

An object of the invention is to provide a method and an apparatusimproved in a manner that

there is no dependence on the thickness of a cable

there occurs no conductance of indicator signal past a faulty location

the current of indicator signal always remains below the currentresisting capacity of a cable.

This object is achieved by means of the invention on the basis of thecharacterizing features set forth in the annexed claims.

The invention will now be described with reference made to theaccompanying drawings, in which

FIGS. 1 and 2 illustrate schematically the operating principle of amethod of the invention.

FIG. 3 shows a block diagram of an apparatus used for generating andtransmitting an indicator signal and

FIG. 4 shows a block diagram of a receiver used receiving an indicatorsignal.

A high-powered electromagnet 1, 2 is used for generating an alternatingmagnetic field, whose lines of magnetic force create a weak signal inthe pair of wires of a cable 3 to be examined. The signal strength canvary from a few uV to a few mV, as the case may be. This weak indicatorsignal produced by the magnetic field is monitored at the outset or atthe end of cable 3 by means of a receiver 4. The present weak signal isamplified in receiver 4 and a confirmation of this is sent to the pointof transmission of this indicator signal. The reception of this"feedback" signal confirms that the cable has an intact pair of wires upto the point said indicator signal is sent from. As a faulty location 9is by-passed, the indicator signal short circuits at said faultylocation and the indicator signal is not received form the outset orfrom the end of a cable. As information of this is continuouslytransmitted by radiophone associated with receiver 4 to a transmissionpoint radiophone 5, the faulty location can be determined at highprecision (0-3 m).

In case a cable appears to have a so-called hole or the like, i.e. thetwin wire is not totally short circuited, the present voltage induced byan external magnetic field is not capable of short circuiting the faultylocation. Thus, a sufficient direct voltage can be supplied from theoutset or the end of a cable for producing a short-circuit current atfaulty location 9. This direct current produced by the direct voltageshort circuiting said faulty location 9 can now be combined with anidentifier or indicator signal generated by means of an externalmagnetic field (magnet 2). A transformer M1 can be used to separate thisweak signal from a current induced by breakdown voltage. The indicatorsignal can also be separated from direct current by other types ofsignal-separation means, such as capacitors C coupled on either side ofa resistance R. Thus, a direct-voltage source 10 is used to generatebetween the wires of cable 3 a sufficient direct voltage for closing acurrent loop at faulty location 9. Thus, a weak indicator signal will becapable of detecting a faulty location 9 even in this type of case,wherein a short circuit in a cable only appears at a rather high voltagewhich is, however, lower than the normal operating voltage. Instead ofdirect voltage it is of course possible to employ a low-frequencyvoltage whose frequency is substantially lower than that of theindicator signal in order to separate the indicator signal therefrom.

An indicator signal transmitter shown in FIG. 3 includes a power source6, e.g., a battery or a like. An AM- or FM-oscillator 7 operates withinthe frequency range of 10 Hz-10 kHz. The employed frequency depends onthe structure of a cable, the type of soil and the burying depth of acable (underground or underwater). The oscillator 7 can also have avariable frequency which requires, however, a correspondingadjustability at the receiver end. Under the control of oscillator 7, apower output stage 8 supplies electric power to a transmitter coil 2.This indicator signal transmitter coil 2 is high-powered, i.e. appr. 5W-200 W. Coil 2 is provided with an iron core for intensifying andaligning the field.

Instead of an electromagnet it is possible to employ e.g. a rotatingpermanent magnet.

FIG. 4 shows a block diagram for a receiver 4 for coupling therewith thetwin wire of a cable 3 to be examined. A weak indicator signal obtainedfrom this twin wire is filtered and amplified by means of a band-passamplifier 11, the output signal obtained therefrom being amplified withan amplifier 12 having an adjustable amplification. The signal obtainedfrom amplifier 12 is carried to a mixer 14, the latter being alsosupplied with an injection frequency from an oscillator 13. Thedifference between these frequencies is carried through a band-passfilter 15 to an amplifier 16. The frequency of oscillator 13 is e.g.twice as high as that of the indicator signal to be received. The signalobtained from amplifier 16 is sent by means of a radio transmitter or aradiophone 17 to a radio receiver or a radiophone 5 placed at thesignal-transmission point, the party searching for a faulty locationthus knowing that the cable is intact up to the point at which the"feedback" signal can be heard. When there is no more signal coming in,nor can it be heard any further away, the fault has been located.

It is obvious that the receiver illustrated in FIG. 4 can be designed inmany other configurations, e.g. by using digital filters for providing anarrower band width and a more sensitive device.

I claim:
 1. A method for locating from above the ground or water a cablefault in a pair of wires in a cable buried underground or underwater,said method comprising transmitting an indicator signal created by amagnetic field, said magnetic field being generated by a magnet locatedat an indicator signal transmission point which induces a current in thewires of a cable (3) to be examined, and monitoring the indicator signalby a receiver (4) connected to the cable, characterized in that betweenthe wires of the cable (3) is supplied a direct voltage which induces acurrent which closes a loop through a faulty location (9), and that theindicator signal is monitored separately from the current produced bysaid direct voltage.
 2. A method as set forth in claim 1, characterizedin that the indicator signal monitored from the cable (3) by receiver(4) is amplified and transmitted back to the indicator signaltransmission point to indicate whether or not an indicator signal isbeing monitored.
 3. A method as set forth in claim 1 or 2, characterizedin that the method is applied for locating an underground-buried cablefrom above the ground.
 4. The method of claim 1, wherein the magnetcomprises a high powered electromagnet or a rotating magnet.
 5. Anapparatus for locating from above the ground or water the position of orcable fault in a cable buried underground or underwater, said apparatuscomprising means (1, 2) for generating an alternating magnetic field anda receiver (4) connectable to the wires of the cable (3) to be examined,said receiver being adapted to filter and amplify an indicator signalwhich is induced in cable wires by said alternating magnetic field at asignal transmission point, characterized in that said apparatus includesa direct-voltage source (10) for supplying a direct voltage between thewires of the cable (3) as well as a signal-separating means (M1; R, C)for separately monitoring the indicator signal from a direct currentproduced by the voltage source (10).
 6. An apparatus as set forth inclaim 5, characterized in that said receiver (4) is provided with atransmission means and the indicator signal transmission point isprovided with a receiving means, whereby an indicator signal passingalong the examined cable (3) to receiver (4), or a signal derivedtherefrom, is an audio signal.
 7. The apparatus of claim 6, wherein thetransmission means comprises a radio transmitter and the receiving meanscomprises a radio receiver.
 8. The apparatus of claim 6, wherein thetransmission means and the receiving means each comprise a radiophone.